One of my favorite tools for squaring up HST blocks is a square ruler with a diagonal line. (There are lots of nifty tools for such purposes.) You don’t need to have this – it’s definitely possible to square-up with any ruler – but if your quilting budget allows for one of these, over time it’s a great investment.
First trim two sides, then rotate the ruler so that the 2 1/2″ lines (in this case) are on the newly squared-up edges and square-off the other two edges. I know it’s a little more tedious to square off all 4 sides, but it definitely makes that block neat with straight edges on all four sides.
If the block has fairly square sides before sewing up, you could place the 2 1/2″ lines directly on the left and bottom edges (right and bottom if you’re left handed) and just square-off the right and top edges. Your call. Make sure you trim those little ‘tags’ in the corner though, so you won’t have a lot of bulk when assembling your block.
Post Edit: I have since found a new favorite short-cut method for squaring-up HST’s using a specialty ruler.
Hope that helps!